Here
is the latest and greatly improved TX200 VFO/VCO
FM transmitter. The most versatile transmitter to
date that can be turned into high fidelity stereo
PLL based 200mW FM transmitter. It is a perfect
circuit for transmitting your music around the house
and yard. TX200 uses only two coils; one in the
oscillator and the other one in the 200mW VHF amplifier
so it should be fairly easy for anyone to build.
It also includes built-in pre-emphasis and C5 for
enhanced sound quality. While assembling the transmitter
care must be taken to make sure that C1 is directly
connected to L1 and C9 to L2. These caps eliminate
the distortions form the DC supply and improve the
sound quality greatly. 9V voltage supply is also
very important because it provides the exact amount
of current to Q1 to produce loud and clear sound
quality. I hope that you'll have as much fun as
I had while building this transmitter. Enjoy! ;)

Power
Meter

TX200
comes with built-in LED based power meter. This
is a very helpful tool that will tell you if your
transmitter's oscillator is working properly.
If RF signal is transmitted the LED will illuminate.
Besides that, it will also give you a quick visual
way to check how much power is being transmitted.
I highly recommend that you have the transmitter
and power meter on the same PCB. If you like to
experiment a lot, you will appreciate this inexpensive
but externally helpful addition.

How
to use PLL to digitally tune the frequency of the TX200 transmitter?

If
you want to use Phase Locked Loop (PLL) to digitally
tune the frequency of TX200 then you will need
to make small changes to the transmitter's oscillator.
Theoretically, change original variable frequency
oscillator (VFO) to voltage controlled oscillator
(VCO). PLL uses voltage to change the capacitance
of the varicap diodes / frequency of the oscillator
so two BB105 varicap diodes or any 15-30pF varicaps
will be perfect for this.

You vary the capacitance of the varicap diode(s)
by applying voltage supply between 1 - 8V. If
you apply 1V the capacitance will be around
34pF, and if you apply 8-30V the capacitance
will be 12pF (minimum). Now, you may wonder
why do we need to start from 1V? The reason
is that if you apply voltages below 1V the capacitance
may go as high as 200pF and we simply can't
use that. 100K resistor will be used to lower
the current of the voltage, because varicaps
require only 0.2mA of current to operate!

With this knowledge we replace C6 - 15pF capacitor
with two varicaps and one resistor. That much
is enough to be able to use 1 - 8V voltage to
change the oscillator's frequency. The second
change that we will need to make is to use 2pF
capacitor and connect it to the Q2 transistor's
collector. PLL will use this to sample the frequency
and compare it with digitally selected frequency.
PLL will then compare those frequencies and
if they are not the same PLL with vary the voltage
that is applied to the varicaps, until those
two frequencies are the same.

Before you even connect PLL circuit to the TX200
transmitter you will need to test if the oscillator
is working properly. We can tune through entire
FM band (88-108MHz) by applying the voltage
to the varicaps. To do so, connect "PLL
- Voltage Control" PIN to the 10K potentiometer
as shown below. With this setup you will be
able to use potentiometer to change the frequency
of the transmitter. If you can't go through
the entire FM band or if your frequency is out
completely then you will need to change the
inductance of the L1 coil.

PLL Module

The schematic and instructions
for building this PLL module can be found at
pira.cz.
After building it and doing some testing with
it I was amazed by how well this Phase Locked
Loop circuit performs. It's stable, compact,
very easy to build and connecting it to TX200
transmitter was a piece of cake!

Make
TX200 a Stereo Transmitter!

You can easily turn TX200 into
a high quality stereo transmitter by connecting
this stereo encoder module that you can either
build
it yourself or buy
it as a kit. Stereo encoder kit
includes all the hard to find components such
as NJM2035 IC and 38KHz crystal.

TX200
Prototype Images

Where to get the Parts

If you are building the above FM Transmitter and have trouble finding components such as variable coils, BF199 transistors or ferrite beads we are distributing these components in our Electronics-DIY store.

Build your own Accurate LC Meter (Capacitance Inductance Meter) and start making your own coils and inductors. This LC Meter allows to measure incredibly small inductances making it perfect tool for making all types of RF coils and inductors. LC Meter can measure inductances starting from 10nH - 1000nH, 1uH - 1000uH, 1mH - 100mH and capacitances from 0.1pF up to 900nF. The circuit includes an auto ranging as well as reset switch and produces very accurate and stable readings.

Volt Ampere Meter measures voltage of 0-70V or 0-500V with 100mV resolution and current consumption 0-10A or more with 10mA resolution. The meter is a perfect addition to any power supply, battery chargers and other electronic projects where voltage and current must be monitored. The meter uses PIC16F876A microcontroller with 16x2 backlighted LCD.

Frequency Meter / Counter measures frequency from 10Hz to 60MHz with 10Hz resolution. It is a very useful bench test equipment for testing and finding out the frequency of various devices with unknown frequency such as oscillators, radio receivers, transmitters, function generators, crystals, etc.

ESR Meter kit is an amazing multimeter that measures ESR values, capacitance (100pF - 20,000uF), inductance, resistance (0.1 Ohm - 20 MOhm),
tests many different types of transistors such as NPN, PNP, FETs, MOSFETs, Thyristors, SCRs, Triacs and many types of diodes.
It also analyzes transistor's characteristics such as voltage and gain. It is an irreplaceable tool for troubleshooting and repairing electronic equipment by determining performance and health of electrolytic capacitors. Unlike other ESR Meters that only measure ESR value this one measures capacitor's ESR value as well as its capacitance all at the same time.

Arduino Prototype is a spectacular development board fully compatible with Arduino Pro. It's breadboard compatible so it can be plugged into a breadboard for quick prototyping, and it has VCC & GND power pins available on both sides of PCB. It's small, power efficient, yet customizable through onboard 2 x 7 perfboard that can be used for connecting various sensors and connectors. Arduino Prototype uses all standard through-hole components for easy construction, two of which are hidden underneath IC socket. Board features 28-PIN DIP IC socket, user replaceable ATmega328 microcontroller flashed with Arduino bootloader, 16MHz crystal resonator and a reset switch. It has 14 digital input/output pins (0-13) of which 6 can be used as PWM outputs and 6 analog inputs (A0-A5). Arduino sketches are uploaded through any USB-Serial adapter connected to 6-PIN ICSP female header. Board is supplied by 2-5V voltage and may be powered by a battery such as Lithium Ion cell, two AA cells, external power supply or USB power adapter.

Having the ability to control various appliances inside or outside of your house wirelessly is a huge convenience, and can make your life much easier and fun. RF remote control provides long range of up to 200m / 650ft and can find many uses for controlling different devices, and it works even through the walls. You can control lights, fans, AC system, computer, printer, amplifier, robots, garage door, security systems, motor-driven curtains, motorized window blinds, door locks, sprinklers, motorized projection screens and anything else you can think of.